1. Field of the Invention
The present invention relates to certain polyurethanes which contain acryloyl groups and can be converted into crosslinked products by radiation curing and to a process for their preparation.
2. Description of the Prior Art
Polyurethanes which contain acryloyl groups and are known and may be obtained, as described, for example, in DE-OS (German Published Specification) 1,644,798, DE-OS 2,115,373 or DE-OS 2,737,406, as the reaction product of, inter alia, polyisocyanates and hydroxyalkyl acrylates. They are cured with the aid of high-energy rays, such as UV rays or electron beams, and are used for coating, for example, wood or for the production of coatings for graphical purposes.
A wide variety of polyisocyanates can be employed for the preparation of the urethane acrylates. If the production of surface coating films which are resistant to yellowing is desired, (cyclo)aliphatic polyisocyanates are employed for the preparation of the urethane acrylates. Although hexamethylene diisocyanate, inter alia, is mentioned in DE-OS (German Published Specification) 2,115,373 as a suitable starting diisocyanate, it can be said that this is a purely hypothetical disclosure, since prior to the present invention it has not been possible to employ this inexpensive diisocyanate in practice because of the crystallinity of the urethane acrylates formed, which prohibits their use as surface coating binders.
Therefore, an object of the present invention is to provide aliphatic polyurethanes containing acryloyl groups which are prepared from hexamethylene diisocyanate (HDI) as the essential starting diisocyanate and which nevertheless are liquid and crosslink rapidly under UV irradiation.
This object has been achieved by the aliphatic polyurethanes containing acryloyl groups according to the invention which are described in more detail below.